machine: add missing doc for memory-backend option

Add documentation for '-machine memory-backend' CLI option and
how to use it.

And document that x-use-canonical-path-for-ramblock-id,
is considered to be stable to make sure it won't go away by accident.

x- was intended for unstable/iternal properties, and not supposed to
be stable option. However it's too late to rename (drop x-)
it as it would mean that users will have to mantain both
x-use-canonical-path-for-ramblock-id (for QEMU 5.0-5.2) versions
and prefix-less for later versions.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20210121161504.1007247-1-imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/backends/hostmem.c b/backends/hostmem.c
index be0c3b079f..c6c1ff5b99 100644
--- a/backends/hostmem.c
+++ b/backends/hostmem.c
@@ -494,6 +494,16 @@ host_memory_backend_class_init(ObjectClass *oc, void *data)
         host_memory_backend_get_share, host_memory_backend_set_share);
     object_class_property_set_description(oc, "share",
         "Mark the memory as private to QEMU or shared");
+    /*
+     * Do not delete/rename option. This option must be considered stable
+     * (as if it didn't have the 'x-' prefix including deprecation period) as
+     * long as 4.0 and older machine types exists.
+     * Option will be used by upper layers to override (disable) canonical path
+     * for ramblock-id set by compat properties on old machine types ( <= 4.0),
+     * to keep migration working when backend is used for main RAM with
+     * -machine memory-backend= option (main RAM historically used prefix-less
+     * ramblock-id).
+     */
     object_class_property_add_bool(oc, "x-use-canonical-path-for-ramblock-id",
         host_memory_backend_get_use_canonical_path,
         host_memory_backend_set_use_canonical_path);